The present invention relates to a white balance adjusting system for a color image pickup apparatus, for example, a color video camera.
As described in Japanese Patent Application Laid Open No. 63-109686, the time-wise response characteristics of white balance adjusting against color temperature variation of illumination in conventional white balance adjusting system are determined by time constant of low pass filter which integrates a color-difference signal inside the white balance adjusting circuit and by counter-drive pulse cycle, while these response characteristics remain fixed.
In addition, as described in U.S. Pat. No. 4,797,733, conventional systems extract the signal for a portion which is considered as white (or achromatic) in the image picture and control the gain of red color signal R and blue color signal B in the signal path, so that the ratio between red and blue color signals R and B in an averaged signal for this portion will be equal (R-B=0) and that it will be parallel to the locus of color temperature variation according to blackbody radiation. FIG. 6 is a vector diagram of chrominance signal. In FIG. 6, the locus of color temperature variation due to blackbody radiation is shown using a thick full line. Furthermore, in such conventional system, generation of control signal for white balance adjusting had been performed by comparing the averaged signal (voltage) of the white portion with a reference signal (voltage) corresponding to R-B (=0) at a certain color temperature.